Optical modulators have been employed for many years in the field of optical communications to accept modulated data in electrical format (typically radio frequency or RF) and transfer the data onto an optical carrier. In a Mach-Zehnder optical modulator 20, as generally shown in FIG. 1A (PRIOR ART), a beamsplitter 22 divides the laser light from an input optical waveguide 24 into two optical beams propagating in parallel waveguides defining optical paths 28A and 28B, at least one of which having a phase modulator in which the refractive index is a function of the strength of the locally applied electric field. In the example of FIG. 1A light in both optical paths 28A, 28B undergoes a phase modulation, although in other configurations the refractive index in only one of the optical paths could be modulated with respect to the other. The beams are then recombined by an output optical combiner 26. Changing the electric field on the phase modulating paths determines whether the two beams interfere constructively or destructively when recombined, and thereby controls the amplitude or intensity of the exiting light. In some configurations, the phase of the exiting light can be controlled via a variety of means such as by manipulating the phase modulation signal, or through design.
In the configuration shown in FIG. 1A, the modulating electric field is provided by a segmented travelling wave electrode 21 (or TWE) that consists of two or more transmission line conductors 30A, 30B oriented substantially parallel to the optical paths 28A, 28B, and a plurality of pairs of waveguide electrodes 32A, 32B. Each waveguide electrode 32A, 32B is connected one of the transmission line conductors 30A, 30B via a corresponding tap or bridge conductor 34A, and 34B. Each bridge conductor 34A, 34B branches out of one of the transmission line conductors 30A, 30B in a direction substantially perpendicular to the optical path 28A, 28B. The transmission line conductors 30A, 30B convey an RF signal along an RF path that is substantially parallel to the optical paths 28A, 28B.
The configuration shown in FIG. 1A is known as a Mach-Zehnder modulator operated in “push-pull” mode is referred to as a series push-pull travelling wave electrode, after Klein et al., “1.55 μm Mach-Zehnder Modulators on InP for optical 40/80 Gbit/s transmission networks”, OFC/NFOEC 2006, paper TuA2, and described in further detail by R. G. Walker, “High-Speed III-V Semiconductor Intensity Modulators”, IEEE J. Quant. Elect., vol. 27(3), pp. 654-667, 1991. In a series push-pull configuration, a single voltage signal or field is used to phase modulate the interfering signals in the two arms in anti-phase. Each pair of waveguide electrodes 32A, 32B, as shown in FIG. 1A, impart a phase change to the optical wave in the waveguide 28A, 28B and also act as a pair of capacitors in series and as a load on the main transmission line conductors 30A, 30B.
A travelling wave electrode Mach-Zehnder optical modulator can be driven using a single RF signal input, as illustrated in FIG. 1B (PRIOR ART). In the illustrated example, the travelling wave electrode 21 of the modulator 20 includes a first transmission line conductor conveying the input electrical signal, therefore acting as a signal transmission line conductor (S), and a second transmission line conductor connected to a ground reference, therefore acting as a ground transmission line conductor (G). This modulator configuration is single-end as it includes a single signal transmission line and is sometimes referred to as an SG modulator (also known as coplanar strip). In the specific embodiment shown in FIG. 1B, the electrical modulation signal is provided by an RF voltage source 50 having a single signal line 52 and a ground line 54, both embodied by a RF waveguide such as a coaxial cable. The signal line 52 of the driver 50 is connected to the signal transmission line conductor S of the travelling-wave electrode 21, whereas the ground line 54 of the driver 50 is connected to the ground transmission line conductor G of the travelling-wave electrode 21. A nominal terminal load 56 (e.g., 50 ohms) joins the distal ends of the S and G transmission lines. The modulation voltage across the arms of the travelling wave electrode is the difference between the signal voltage and ground.
It should be noted that other types of RF drives are known in the optical telecommunications industry, requiring other arrangements of transmission line conductors in the modulator. For example, the prior art includes optical modulators with differential-drive GSGSG and GSSG formats.
In some applications, single end driving may be advantageous over other types of drive, such as differential drive. Single-end travelling wave electrode Mach-Zehnder optical modulators are known in the art to provide broadband high frequency operation. In addition, a single-end drive can reduce the size of packaging, since only one high-frequency signal feed-through is necessary to connect to the optical modulator. Routing of differential signals can indeed be challenging, as it is preferred that the two opposite signals that comprise the differential signal be of opposite sign, but otherwise identical.
The SG format of prior art single end travelling wave electrode optical modulators, however, suffers from a major drawback. The signal transmission line conductor S is not shielded from the external environment by a grounded conductor. Parasitic interaction between the signal and the environment can negatively impact performance. A large buffer distance is required between the optical modulator and any neighboring environmental features (e.g., metal, dielectric interfaces, etc.), especially on the side of the exposed signal conductor. If the optical modulator is monolithically integrated with other components, this places a limit on how densely the components can be placed. Dense integration is desirable because it enables miniaturization and reduced manufacturing costs.
There remains a need, therefore, for a modulator configuration that alleviates at least some of the drawback of the prior art.